A fibre Bragg grating (FBG) is a permanent periodic refractive index modulation in the core of a single-mode optical silica glass fibre over a length of typically 1–100 mm. It can be created in a photosensitive fibre by transversely illuminating the fibre with a periodic interference pattern generated by ultra-violet (UV) laser light. The refractive index modulation in a standard FBG is believed to form by UV induced breaking of electronic bonds in Ge-based defects, releasing electrons which are re-trapped at other sites in the glass matrix. This causes a change in the absorption spectrum and in the density, and thereby a change in the refractive index of the glass. An FBG reflects light within a narrow bandwidth (typically 0.1–0.3 nm), centred at the Bragg wavelength, 1B=2 neffL, where neff is the effective refractive index seen by the light propagating in the fibre, and L is the physical period of the refractive index modulation. It is known that the reflected Bragg wavelength from an FBG will change with any external perturbation which changes the effective refractive index seen by the propagating light and/or the physical grating period (fibre length), such as temperature and strain. By measuring the reflected Bragg wavelength, using for example a broadband light source and a spectrometer, an FBG can be used as a sensor for measuring such external perturbations. A standard UV induced FBG can be made thermally stable up to ca. 300–400° C., at higher temperatures the UV-induced refractive index modulation will decay fast and the grating will be erased.
It is possible to make so-called chemical FBGs which can survive temperatures up to 1100–1200° C. [Fokine, M., Sahlgren, B. E., and Stubbe, R., “High temperature resistant Bragg gratings fabricated in silica optical fibres,” ACOFT-96, post-deadline-paper, 1996, Sidney, Australia and PCT patent application WO 98/12586 to Fokine]. A chemical grating is typically formed by first writing a standard grating in hydrogen loaded, fluorine (F) co-doped, Ge-doped silica fibres. UV exposure of such a fibre creates OH in the illuminated regions of the fibre which through heating reacts with F to form HF. Post-annealing at temperatures >1000° C. causes the HF to diffuse out of the fibre core, leaving UV-exposed areas more depleted of F than unexposed areas, producing a spatially varying F-concentration and hence a refractive index variation (grating). It is also possible to make other types of special FBGs, which can survive high temperatures, such as type II FBGs [W. X. Xie et.al., Opt. Commun. 1993, Vol. 104, pp. 185–195]. It is known that type II FBGs in germanium-free nitrogen-doped silica-core fibres are much more stable at elevated temperatures than standard type I FBGs [E. M. Dianov et.al., Electron. Lett., Vol. 33, pp. 236–237, 1997].
Several FBGs can be wavelength multiplexed along one fibre, making them very attractive for distributed measurements of strain and temperature. FBGs can also be used as a pressure sensor by measuring the shift in Bragg wavelength caused by hydrostatic pressure induced compression of the silica glass fibre [Xu, M. G., Reekie, L., Chow, Y. T., and Dakin, J. P., “Optical in-fibre grating high pressure sensor, Electron. Lett., Vol. 29, pp. 398–399, 1993]. This provides a very simple sensor design with small dimensions and good reproducibility and long-term stability provided by the all-silica construction of the sensor. An all-fibre FBG sensor with enhanced pressure sensitivity and inherent temperature compensation can be made by using a passive or an active (fibre laser) FBG written in a birefringent side-hole fibre, which has two open channels symmetrically positions at each side of the fibre core, [Udd, E., U.S. Pat. Nos. 5,828,059 and 5,841,131, Kringlebotn, J. T., Norwegian patent application 19976012 (passive FBG sensors) and Kringlebotn, J. T., U.S. Pat. No. 5,844,927 (active FBG sensor)]. It is also possible to make FBG pressure sensors with enhanced pressure sensitivity by using a glass transducer element surrounding the optical fibre, either to convert pressure to strain/compression in the fibre or to convert pressure to fibre birefringence [Udd, E., U.S. Pat. No. 5,841,131].
When fibre-optic sensors are operated under conditions of high temperature, such as in oil wells, there might be considerable drift effects both in FBG and birefringent interferometric sensors, as taught us by J. R. Clowes et.al. in “Effects of high temperature and pressure on silica optical fibre sensors,” IEEE Photon. Technol. Lett., Vol. 10, pp. 403–405, 1998. The drift effect occurs when the fibre is surrounded by a liquid, such as water or oil, and increases with increasing temperature. The effect is believed to be due to ingress of liquid molecules into the outer layers of the fibre cladding resulting in the development of a highly stressed layer and consequently a tensile stress on the fibre core. This increases the optical path length of a fibre and changes the Bragg wavelength of an FBG. This effect will also change the birefringence of a highly-birefringent fibre. Clowes et.al. demonstrated that the increase in optical path length of a fibre was reduced by an order of magnitude using a hermetic, carbon coated fibre.
In addition, diffusion of gases, such as hydrogen, into the fibre, will cause a change in the refractive index proportional to the hydrogen concentration, and consequently drift in Bragg wavelength of an FBG written into the core of the fibre, as disclosed by Malo et.al. in “Effective index drift from molecular hydrogen diffusion in hydrogen-loaded optical fibres and its effect on Bragg grating fabrication,” Electronics Letters, Vol. 30, pp. 442–444, 1994. Hydrogen will also cause a loss increase in an optical fibre, which could be detrimental for FBG-based rare-earth doped fibre lasers. Finally, diffusion of gases into the holes of a side-hole fibre will change the pressure inside the holes, and hence the pressure difference which affects the measurement of the external hydrostatic pressure.
As disclosed by Kringlebotn in Norwegian patent application 19976012 a practical all-fibre FBG pressure sensor without drift at high-temperature operation can be realised by recoating an FBG in a side-hole fibre with a hermetic coating to prevent penetration of gases, vapours or liquids from the surrounding environment. However, there is no mentioning of how such a coating can be applied on an FBG.
A. Hay [U.S. Pat. No. 5,925,879] discloses the use of a carbon coating, or another hermetic coating on an FBG sensor to protect the optical fibre and sensors from a harsh environment.
Carbon has been shown to provide a good hermetic coating for optical fibres, making them essentially impermeable to both water and hydrogen, maintaining the mechanical strength and low loss of the fibre. A carbon coating can be applied to an optical fibre during the drawing process before the fibre glass cools through a pyrolytic process [see for example U.S. Pat. No. 5,000,541 to DiMarcello et.al.]. Carbon coating using a similar technique can also be applied to splices between hermetic fibres to maintain hermeticity after splicing of carbon coated fibres [U.S. Pat. No. 4,727,237 to Schantz, C. A., et.al.]. In the latter patent a pyrolytic technique is used based on heating the fibre splice region with a CO2-laser inside a chamber containing a reactant gas causing a carbon coating to form on the glass surface by pyrolysis of the reactant gas. The temperature in the fibre will during such a process typically exceed 1000° C. This high temperature pyrolytic process have been shown to provide highly hermetic coatings, and seems to be the preferred technique for carbon coating of optical fibres. However, a standard FBG, i.e. a so-called type-I FBG in a germanium-doped silica fibre cannot be carbon coated using such a process, since it will be erased at the high temperature involved.